feat: message reactions + link/image previews

Reactions (full stack):
- Wire: MsgReaction peer message (reaction_mid, reaction_emoji)
- Store: reactions table (mid, emoji, from_peer), SaveReaction, ReactionsForRoom
- Daemon: CmdSendReaction IPC command; EvtReaction IPC event; stored reactions
  replayed to newly-connected IPC clients alongside history
- Web UI: reactions state (mid → emoji → [fromId]); hover a message to reveal
  a + button; click opens a 6-emoji picker (👍❤️😂😮😢🙏); reaction chips
  appear below the message with count and tooltip; own reactions highlighted

Link/image previews:
- URLs in message text rendered as clickable <a> links
- Image URLs (.jpg/.jpeg/.png/.gif/.webp/blob:/data:image) rendered as inline
  thumbnails (max 320×200px) below the link

Also scoped push notifications architecture in FUTURE.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/store/store.go

@@ -38,10 +38,19 @@ CREATE TABLE IF NOT EXISTS rooms (
 
 // migrations run after the base schema. ALTER TABLE ADD COLUMN fails with
 // "duplicate column name" on subsequent opens — we swallow that error.
+// CREATE TABLE IF NOT EXISTS and CREATE INDEX IF NOT EXISTS are idempotent.
 var migrations = []string{
 	// EXT-007: canonical message ID for history dedup (NULL for pre-feature messages).
 	`ALTER TABLE messages ADD COLUMN msg_id TEXT`,
 	`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_msg_id ON messages (msg_id) WHERE msg_id IS NOT NULL`,
+	// Reactions: (mid, emoji, from_peer) triple is the unique dedup key.
+	`CREATE TABLE IF NOT EXISTS reactions (
+		mid        TEXT NOT NULL,
+		emoji      TEXT NOT NULL,
+		from_peer  TEXT NOT NULL,
+		reacted_at DATETIME NOT NULL,
+		PRIMARY KEY (mid, emoji, from_peer)
+	)`,
 }
 
 // Store is a local SQLite-backed message and peer store.
@@ -208,6 +217,43 @@ func (s *Store) KnownPeers() (map[proto.PeerID]string, error) {
 	return out, rows.Err()
 }
 
+// SaveReaction persists a reaction. Duplicate (mid, emoji, from_peer) triples are silently ignored.
+func (s *Store) SaveReaction(mid, emoji, fromPeer string) error {
+	_, err := s.db.Exec(
+		`INSERT OR IGNORE INTO reactions (mid, emoji, from_peer, reacted_at) VALUES (?, ?, ?, ?)`,
+		mid, emoji, fromPeer, time.Now().UTC(),
+	)
+	return err
+}
+
+// ReactionsForRoom returns all reactions for messages in a given room.
+// Result: mid → emoji → []fromPeer (ordered by reaction time).
+func (s *Store) ReactionsForRoom(room string) (map[string]map[string][]string, error) {
+	rows, err := s.db.Query(`
+		SELECT r.mid, r.emoji, r.from_peer
+		FROM reactions r
+		JOIN messages m ON m.mid = r.mid
+		WHERE m.room = ?
+		ORDER BY r.reacted_at
+	`, room)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := make(map[string]map[string][]string)
+	for rows.Next() {
+		var mid, emoji, from string
+		if err := rows.Scan(&mid, &emoji, &from); err != nil {
+			return nil, err
+		}
+		if out[mid] == nil {
+			out[mid] = make(map[string][]string)
+		}
+		out[mid][emoji] = append(out[mid][emoji], from)
+	}
+	return out, rows.Err()
+}
+
 func nullableString(s string) any {
 	if s == "" {
 		return nil